1. Field of the Invention
The present invention relates to a vacuum chemical vapor deposition (referred to as CVD hereinafter) apparatus and a method of cleaning the CVD apparatus. More particularly, the present invention is concerned with a vacuum CVD apparatus used for forming by deposition a polycrystalline silicon film in semiconductor production process and also to a method for cleaning the vacuum CVD apparatus.
2. Description of the Related Art
FIG. 4 schematically shows the construction of a conventional horizontal-type CVD apparatus. Referring to this Figure, a plurality of semiconductor wafers, on each of which a polycrystalline silicon film is to be formed, are mounted on a quartz boat 2 which is received in a reaction chamber defined by, for example, a quartz tube 3. Vacuum flanges 4a, 4b are provided on both ends of the quartz tube 3. The quartz tube 3 is surrounded by a heater 5 for controlling the reaction temperature.
Deposition of the polycrystalline film is conducted by using, for example, SiH.sub.4 gas, together with PH.sub.3 gas which is used to dope the polycrystalline silicon film with phosphorus for the purpose of controlling the resistivity of the polycrystalline film. At the same time a carrier gas such as N.sub.2 gas is used. These gases are supplied, respectively, from an SiH.sub.4 gas source 6, a PH.sub.3 gas source 7 and an N.sub.2 gas source 8, via flow-rate controlling mass-flow controllers 9, 10 and 11 and valves 12, 13 and 14, such as pneumatically operated valves.
Each of these gases is supplied to the quartz tube 3 through either one of the vacuum flanges 4a and 4b on both sides of the quartz tube 3, by the actions of gas introduction valves 15a, 15b such as pneumatically operated valves which change the direction of introduction of the gas in a manner like a flip-flop. The gas, after use in the quartz tube 3 is discharged therefrom by a vacuum pump, e.g., a dry pump 19, via one of gas discharge valves 16a, 16b which are provided in a vacuum pipe 16A and operatively associated with gas introduction valves 15a, 15b, located beyond a vacuum main valve 17 and a sub-valve 18.
The term "manner like a flip-flop" is intended to mean such a switching operation that the following first and second phases of operation are switched at a predetermined period. In the first phase, the gas introduction valve 15a and the gas discharge valve 16b are opened to allow the gas to be introduced into the quartz tube 3 through the vacuum flange 4a and discharged through the vacuum flange 4b, whereas, in the second phase, the gas introduction valve 15a and the gas discharge valve 16b are closed and, instead, the gas introduction valve 15b and the gas discharge valve 16a are opened to allow the gas to be introduced into the quartz tube 3 through the vacuum flange 4b and discharged through the vacuum flange 4a.
In the conventional horizontal-type vacuum CVD apparatus, having the described construction, phosphorus-doped polycrystalline silicon film is deposited by using SiH.sub.4 gas, PH.sub.3 gas and N.sub.2 gas, by the flip-flop type gas introduction-discharge switching system. The PH.sub.3 gas is diluted by, for example, Ar gas at a PH.sub.3 /Ar ratio of 1/99, the Ar gas being supplied from the PH.sub.3 gas source 7.
The deposition of the phosphorus-doped polycrystalline silicon film on the silicon wafer 1 is conducted as follows. The quartz boat 2 carrying a plurality of silicon wafers 1 is set in the quartz tube 3 under atmospheric pressure. Then, the vacuum sub-valve 18 is opened and the dry pump 19 is started to gently reduce the pressure inside the quartz tube 3 down to 20 Torr or so. The pressure reduction has to be done gently in order to prevent evolving of particles which reside on the quartz tube 3, thereby preventing defects in patterns formed on the silicon wafers which may otherwise be caused due to the deposition of such particles.
When the pressure inside the quartz tube 3 has come down to below 20 Torr, the vacuum discharging main valve 17 is opened to further reduce the pressure down to 10.sup.-3 Torr. When this reduced pressure is reached, the SiH.sub.4 gas, PH.sub.3 gas and the N.sub.2 gas are introduced into the quartz tube 3 through the gas introduction pipe 15A, from the SiH.sub.4 gas supply source 6, PH.sub.3 gas supply source 7 and the N.sub.2 gas supply source 8. The flow rates of the gases are controlled by opening the valves 12, 13 and 14 and controlling the mass-flow controllers 9, 10 and 11, such that, for example, the SiH.sub.4 gas supply rate is 800 cc/minute (volume under standard conditions (SCCM), expressed in terms of cc hereinafter), the PH.sub.3 gas supply rate is 150 cc/minute and the N.sub.2 gas supply rate is 300 cc/minute. The flip-flop-type method described above is carried out such that, in the first phase, the gas introduction valve 15a is opened while the gas introduction valve 15b is closed so that the gas is introduced through the gas introduction valve 15a, and the gas discharge valve 16b is opened while the gas discharge valve 16a is closed so that the gas is discharged through the gas discharge valve 16b. Deposition is performed while the pressure inside the quartz tube 3 is maintained at a reduced pressure of 0.6 Torr. Meanwhile, the quartz tube 3 is held at a temperature of about 590.degree. C. by the operation of the heater 5.
The deposition is continued on the silicon wafers for a certain time, e.g., 1 to 2 hours, thereafter the supply of the gases is terminated. Then, the gas introduction valve 15b is opened while the gas introduction valve 15a is closed, and the gas discharge valve 16a is opened while the gas discharge valve 16b is closed, so that the deposition is commenced again by introducing the gases through the gas introduction valve 15b and discharging the same through the gas discharge valve 16a. Thus, the deposition is conducted by alternately using the first and second phases like a flip-flop. This flip-flop type operation is necessary for the purpose of eliminating problems such as variation in the film thickness according to the position of the silicon wafers 1 on the quartz boat 2, as well as lack of uniformity in the film thickness and concentration of P as the dopant in each of the silicon wafers 1, which otherwise is caused when the deposition is continued under unidirectional supply of the gases.
The known horizontal vacuum CVD apparatus suffers from the following problems. A first problem resides in that, since the quartz tube 3 is heated to a high temperature, SiH4 gas is decomposed, with the result that a silicon oxide film, silicon and so forth are deposited on the inner surface of the quartz tube 3. In particular, when the introduction and discharge of the gases are conducted in a flip-flop-like manner as described, silicon oxide film fractions and silicon in the form of micronized particles are blown when the direction of flow of the gases is switched. The blown particles then fall onto the surfaces of the silicon wafers. Such particles undesirably deteriorate reliability of LSIs as the products of the process. In view of the current demand for larger scale of integration of VLSIs, contamination with such particles is becoming a serious problem. Thus, it is a matter of great importance to reduce particles which are undesirably deposited on the silicon wafers 1 in the course of deposition of semiconductor silicon films.
Conventionally, contaminants deposited on the inner surfaces of the quartz tube are removed by, for example, rinsing with fluoro-nitric acid after disconnecting the quartz tube 3 from the piping. This essentially requires suspension of operation of the horizontal CVD apparatus, resulting not only in a reduction of the rate of operation of the apparatus but also in the necessity of laborious cleaning work.